Evaluation of umami taste components of mushroom(Suillus granulatus)of different grades prepared by different drying methods

Xiuhong Zho,Yunyun Wei,Xue Gong,Hern Xu,Gung Xin,*

a Grain College,Shenyang Normal University,Shenyang 110034,China

b College of Food Science,Shenyang Agricultural University,Shenyang 110866,China

ABSTRACT Suillus granulatus is a valuable wild edible mushroom with a strong umami taste.Different grades of S.granulatus were dried by different methods,including natural air drying (ND),hot air drying (HAD),vacuum drying(VD),and freeze drying(FD).Results showed that VD samples of all grades had the highest equivalent umami concentration (EUC) value (P ＜0.05).The second grade of ND samples showed the highest EUC value(P ＜0.05).The first grade of HAD samples showed the highest EUC value(P ＜0.05).The third grade of FD samples showed the highest EUC value(P ＜0.05).The third grade of VD samples had a higher EUC value than the other grades of samples dried by VD.Electronic tongue results indicated no significant differences between samples of all grades dried by all methods.Thus,umami taste components are affected by drying method and grade.VD is an appropriate drying method for all sample grades.ND,HAD,and FD are suitable for second-,first-,and third-grade samples,respectively.

Keywords:Suillus granulatus drying methods grade umami taste components

1.Introduction

Suillus granulatusis a valuable wild mushroom available in China and other countries [1-3].In China,S.granulatusis distributed in northeastern,southwestern,and southern and other regions [4].Due to its characteristics of rich nutrition and strong umami taste,S.granulatusis very popular among consumers.The umami taste characteristics are mainly attributed to nonvolatile taste components,especially free amino acids and 5′-nucleotides[5,6].Mushrooms are favored by consumers for their delicious taste and abundant nutrition.In recent years,as a good source of protein and dietary fiber,mushrooms have been found to possess a variety of active ingredients and health benefits[7-12].

Based on the local classification standards and eating habits of local people,S.granulatusis classified into three grades according to cap diameter.For specific purposes,S.granulatusof different grades were selected.Drying is a common way to extend the shelf life of mushrooms [13,14].It can reduce nutrient loss,inhibit the growth of spoilage microorganisms,and slow down enzyme activity and some moisture-mediated reactions [15,16].Researchers have investigated the effects of drying methods and grades on umami taste active components of various mushrooms[1,14,16-18].It was reported that drying methods significantly affected umami taste components and even promoted the formation of some new taste components [19,20].These effects were mainly due to various chemical reactions occurring during the drying process,especially the Maillard reaction and protein degradation[15,18,21].Cho et al.[18]found that the umami taste active components in pine mushrooms (Tricholoma matsutakeSing.) differed according to grade.However,to the best of our knowledge,reports on umami taste components ofS.granulatusin different grades dried by different drying methods are scarce.

Therefore,the present study aimed to investigate the effects of drying method and mushroom grade on the umami taste components ofS.granulatus.These results will provide a new research direction for studying the umami taste components ofS.granulatusand promote the development of theS.granulatusindustry.

Fig.1.Samples.

2.Materials and methods

2.1.Materials

FreshS.granulatuswere harvested in the Lingyuan area of Liaoning province(Chaoyang City,China).The ITS sequence of this mushroom was determined,and the results were compared by BLAST.Results showed 99% similarity withS.granulatus.All samples were pest-free and without mechanical damage.Samples were classified into three grades according to cap diameter:the first grade(first)(＜4 cm),the second grade(second)(4-8 cm),and the third grade(third)(＞8 cm)(Fig.1 ).Samples were dried by the following methods.Natural drying (ND):samples were dried under conditions of natural wind and sunlight.Hot air drying (HAD):samples were dried in a GL-65B electric blast drying oven(Taisite Instrument Co.,Ltd,Tianjin,China)at a temperature of(60±2)°C.Vacuum drying(VD):samples were dried in a D2F-1B vacuum dryer(Yuejin Medical Instrument Factory,Shanghai,China)at a temperature of (60±2)°C.Freeze drying (FD):samples were dried in an LGJ-12 freeze dryer(Songyuan Huaxing Technology Development Co.,Ltd,Beijing,China)at a temperature of-60°C under a vacuum of 0.1 K Pa.The moisture content was measured every 2 h until it remained constant.Different dried samples were grounded finely.

2.2.Free amino acid determination

The content of free amino acids was determined with an amino acid analyzer L-8900 (L8900; Hitachi Ltd,Japan) following the method of Li et al.[22]with slight modifications.The powder(0.3 g)was diluted with 50 mL of 0.01 mol/L hydrochloric acid and extracted by ultrasonication for 45 min.Subsequently,the suspension was centrifuged at 10000 r/min(11000 g).An equal volume of 8% sulfosalicylic acid was added to the supernatant (5 mL),mixed and placed in the dark for 15 min,and then the suspension was centrifuged at 10000 r/min (11000 g) for 15 min.The final supernatant was filtered through a 0.22-μm mixed cellulose ester filter(Millipore)prior to amino acid analysis.

2.3.5′-Nucleotide determination

Nucleotides were determined by the method of Taylor[23]with some modifications.Mushroom powder(0.5 g)was extracted with 25 mL distilled water and boiled for 1 min.The suspension was centrifuged at 10000 r/min (11000 g) for 20 min.Another 25 mL distilled water was used to extract the nucleotides of the residues as described above.The pooled supernatants were passed through a 0.22-μm membrane filter before high-performance liquid chromatography analysis.

Nucleotides were analyzed with a Waters 1525 instrument(Waters Corporation,Shanghai,China)equipped with a UV detector.The analysis was carried on a Li Chrospher RP-18 column(250 mm×4.6 mm,5 μm,Merck),and the mobile phase was phosphate buffer(pH 4.2)at a flow rate of 1.0 mL/min.The nucleotides were monitored at 254 nm,with an injection volume of 20 μL and an oven temperature of 30°C.

2.4.Equivalent umami concentration(EUC)

The EUC,representing the concentration of monosodium glutamate(MSG),is equivalent to the umami intensity given by a mixture of MSG and 5′-nucleotides and calculated by the following equation[24].

whereYis the EUC of the mixture(g MSG/100 g),aiis the concentration(g/100 g)of each umami amino acid[aspartic acid(Asp)or glutamic acid(Glu)],ajis the concentration(g/100 g)of each umami 5′-nucleotide [5′-guanosine monophosphate (5′-GMP),5′-inosine monophosphate(5′-IMP),5′-xanthosine monophosphate(5′-XMP),or 5′-adenosine monophosphate(5′-AMP)],biis the relative umami concentration (RUC) for each umami amino acid with respect to MSG(Asp,0.077;Glu,1),bjis the RUC for umami 5′-nucelotide to 5′-IMP (5′-IMP,1.0; 5′-GMP,2.3; and 5′-AMP,0.18),and 1218 is a synergistic constant based on the concentration(g/100 g)used.

2.5.Electronic tongue measurement

Electronic tongue analysis was carried on with the Taste-Sensing System SA 402B (Intelligent Sensor Technology,Inc.,Kanagawa,Japan).Different samples were analyzed after the electric potentials of all membranes had been stabilized in standard solutions[25].

2.6.Statistical analysis

All data in our study were detected in triplicate and expressed as the mean±standard deviation.IBM SPSS Statistics version 19(SPSS Inc.,Chicago,IL)was used to carry out statistical analysis and Tukey’s multiple range tests were applied to the mean separations.Differences withP ＜0.05 were regarded as statistically significant.

3.Results and discussion

3.1.Free amino acids

Free amino acids are essential taste active components in mushrooms,especially Asp and Glu,which are very important for the umami taste of mushrooms.Yang et al.[26]classified the umami amino acid content into three levels:high range(＞20 mg/g),middle range (5-20 mg/g),and low range (＜5 mg/g).The contents of 17 free amino acids ofS.granulatusare shown in Table1.It was obvious that the umami amino acid content ofS.granulatusin this study was in the middle range,which had the same tendency as whiteHypsizygus marmoreusdried by four drying methods[27].

Based on taste characteristics,free amino acids can be separated into four classes,MSG-like,sweet,bitter,and tasteless samples(Table1) according to published studies [28].For the first,the total umami amino acids((13.23±0.60)mg/g),sweet amino acids((27.87±1.47)mg/g),and tasteless amino acids((9.77±0.41)mg/g)in ND samples had the highest content of all the drying methods.The highest content of bitterness amino acids((24.24±0.58)mg/g)and total free amino acids ((69.26±1.33)mg/g) were found in FD samples (P＜0.05).For the second,the content of sweet amino acids ((21.38±1.23)mg/g) and total free amino acids((64.48±2.47)mg/g) in ND samples were the highest compared with the other drying methods (P＜0.05).FD samples showed the highest tasteless amino acid ((14.34±0.31)mg/g) content(P＜0.05).Bitterness amino acids((18.86±1.24)mg/g)and umami amino acids((13.52±0.75)mg/g)in FD samples were higher than in those dried by the other methods.For the third,the highest content of free amino acids,including sweetness((21.39±1.37)mg/g),bitterness ((17.52±0.80)mg/g),tasteless ((13.79±0.40)mg/g),and total amino acids((61.93±2.43)mg/g),were found in FD samples,except total umami amino acids,which were the highest in ND samples (P＜0.05).FD samples of all grades had the lowest umami amino acid components,while the content of bitterness amino acids was the highest.This may have been due to interactions between umami and bitterness amino acids[29].

Table1 Effect of drying methods on free amino acids of S.granulatus

Table2 Effect of drying methods on 5′-nucleotide of S.granulatus

Table3 Effect of drying methods on EUC value of S.granulatus

The umami amino acid contents of various grades of samples dried by the same methods were significantly different,in the range from 8.83 mg/g to 16.55 mg/g.The variation trends were as follows:ND:third ＞first ＞second,HAD:second ＞first ＞third,VD:first ＞second ＞third,FD:first ＞third ＞second.The content of sweetness amino acids varied in the following order:ND:first ＞second ＞third,HAD:first ＞third ＞second,VD:second ＞first ＞third,FD:first ＞third ＞second.Samples of the first dried by VD and FD had the highest content of bitterness amino acids,while HAD samples had the lowest.The highest content of total free amino acids in all drying methods was found in the first grade(P＜0.05).Some researchers reported that the final content of free amino acids was not only related to the amount of free amino acid generation from protein degradation but also consumption by the Maillard reaction [30].From our study,we found that free amino acids ofS.granulatuswere related not only to drying method but also to grade.

3.2.Flavor 5′-nucleotides

Flavor 5′-nucleotides,which give a meaty flavor[31],were identified as 5′-GMP,5′-IMP,and 5′-XMP[32].The synergistic effect of flavor 5′-nucleotides and MSG-like components might increase the umami taste of mushrooms to a large extent[24].

As shown in Table2,the flavor 5′-nucleotide content of different grades had the following trends:first:VD＞HAD ＞FD ＞ND,second:VD ＞FD ＞ND ＞HAD,third:VD ＞FD ＞ND ＞HAD.VD obtained the highest value of all grades (P＜0.05).This may have been because HAD took a longer time to reach the same moisture level compared with VD at the same temperature,and the air may degrade the 5′-nucleotide to some extent [33,34].This suggested that a long processing time might affect the content of 5′-nucleotides inS.granulatus.

For the different drying methods,the flavor 5′-nucleotide contents ranked in the following order:ND:first ＞second ＞third,HAD:third ＞first ＞second,VD:second ＞third ＞first,FD:third ＞second ＞first.In HAD and FD samples,the content of flavor 5′-nucleotides in the third were the highest.However,ND samples shown the opposite results:the third had the lowest flavor 5′-nucleotide content.Results of samples dried by VD were different from the results above,and there were no significant differences among the three grades.Yang et al.[26]classified the content of flavor 5′-nucleotides into three levels:high range(＞5 mg/g),middle range (1-5 mg/g),and low range (＜1 mg/g).Therefore,for the most part,the content of flavor 5′-nucleotides in our study belonged to the middle range,except for those of ND and FD samples of all three grades and HAD samples of the second grade.The flavor 5′-nucleotide content ofPleurotus eryngiidried by hot air,vacuum,microwave,FD,and ND were in the low range[26].

3.3.EUC value

Mau et al.[35]divided EUC values into four levels:(1)＞1000 g,(2)100-1000 g,(3)10-100 g,and(4)＜10 g MSG/100 g of dry matter.Based on our experimental data,the EUC values ofS.granulatusvaried widely,ranging from 41.18 g MSG/100 g dry weight FD samples(first)to 512.62 g MSG/100 g dry weight VD mushrooms(third)(Table3).It was apparent that EUC values in our study stood on the boundary between the second and third levels,which was inconsistent with the study of Cho [18].For different grades,the variation in trends were as follows:first:VD ＞HAD ＞ND ＞FD,second:VD ＞FD ＞ND ＞HAD,and third:VD ＞HAD ＞FD ＞ND.

For different drying methods,the EUC value had the following trends:ND:second ＞first ＞third,HAD:first ＞third ＞second,VD:third ＞first ＞second,and FD:third ＞second ＞first.The VD sample showed no significant differences among the three grades.

3.4.Electronic tongue sensory score

The electronic tongue was used for sensory evaluation of mushroom extract [36].As shown in Fig.2,the umami and sweetness sensory scores were higher than the other taste sensory scores.The reason is that the umami and sweet components were the main components that contributed to the natural taste of mushrooms [28].Umami and sweetness sensory scores tested in our study ranged from 8.75 to 11.93 and 8.45 to 13.88,respectively.The highest umami sensory score of all grades were found in ND samples,while the highest sweetness sensory score was found in FD samples(P＜0.05).There were no significant differences in different grade samples dried by the same methods.All samples in our study showed negative scores in sourness and saltiness,which had the same tendency as many mushrooms[25].This result was different from other studies,which showed that the electronic tongue had good consistency with the EUC value[25,37].This may have been due to the mushroom species,the interaction between taste components,and the presence of some taste components which have not been found so far.The causes await further study in the future.

4.Conclusions

The highest EUC values of all grades were found in VD samples.The umami taste sensory score of ND samples in all grades were higher than those dried by other drying methods.For drying methods,the third grade of ND samples was the highest in different grades.The first grade of HAD samples were the highest of the different grades.The third grade of FD samples was the highest of the different grades.The third grade of VD samples had the highest EUC value of all other grades,but there were no significant differences between the various grades.The electronic tongue results indicated no significant differences among samples of all grades dried by all methods.In comparisons between EUC values and electronic tongue results,some differences did exist.The reason for these differences awaits further study.Given the EUC value,VD was more suitable for preservation of the umami taste components of allS.granulatus.ND was a good choice for second-grade samples,HAD was a good choice for first-grade samples,and FD was a good choice for third-grade samples.These results will provide a theoretical basis for carrying out further research on the mechanism of the effect of different drying methods on umami taste components ofS.granulatusand promote the development of the mushroom industry.

Fig.2.Spider plot for the sensory score based on the taste sensing system of S.granulatus.

Declaration of Competing Interest

The authors declare no conflict of interests.

Acknowledgements

Liaoning Provincial Department of Education Annual(2019)Scientific Research Fund Project (No.LSNZD201903); National Key R&D Program of China(No.2018YFD0400200);Liaoning Province,Shenyang Agricultural University,High-end Talent Introduction Fund Project(NO.SYAU20160003),and Natural Science Foundation of Liaoning Province(No.20170540822).